Optical disk recording devices include an objective lens which focuses a laser light beam onto an optical disk while the disk is rotating. Both focus and tracking of optical disks must continuously be maintained while the disks are recording. Focus and tracking actuators are typically used in optical disk recording and playback devices to control the position of the objective lens relative to the rotating disk. Focus is controlled by adjusting the distance between the objective lens and the disk surface. Tracking is controlled by correcting the radial position of the objective lens so that a single data track is read or written at a time. The motions of the actuator in the focus and tracking directions are conventionally controlled by a servo system, and the forces which cause these motions are typically generated by one or more electromagnetic motors.
Alignment of the optical axis of the objective lens to the optical disk must be maintained accurately for the system to function properly. Higher data densities in optical recording require higher quality optics. Any tilt of the disk surface relative to the optical axis of the objective lens decreases the spot quality at the recording layer on the disk. The tilt of the disk surface introduces a wavefront aberration called coma. The coma deteriorates the ability to read or write marks on the disk. In addition, the disk tilt can introduce an offset in the tracking signal.
FIG. 1 shows a conventional optical actuator generally designated 10. The optical actuator 10 includes an objective lens 12, which is well known in the art to focus a light beam (not shown) onto the surface of an optical disk (not shown). The objective lens is fixed to a lens holder 14 by an adhesive or other fastening member (not shown). The optical actuator 10 further includes four spaced-apart flexures 18 to connect the lens holder 14 to a base 16. A conventional motor is shown, which includes a pair of magnets 20, which are fixed to a pair of magnetic flux yokes 22 secured to opposite sides of the base 16. The motor further includes a focus coil 24 and four tracking coils 26 which are bonded to the lens holder 14. The focus coil 24 and tracking coils 26 are positioned with the magnetic flux generated by the magnets 20 and the magnetic flux yokes 22. The focus coil 24 and tracking coils 26 provide forces which deform the flexures 18 and move the lens holder 14 in the Y and Z directions so as to maintain the focus and tracking of the laser beam (not shown) on the optical disk (not shown). In operation, current is applied to the focus coil 24 to create a magnetic field. By energizing the focus coil 24, the lens holder 14 moves in a direction which is perpendicular to the surface of the optical disk. By energizing the tracking coils 26, the lens holder 14 moves in a direction parallel to the plane of the optical disk for tracking of the optical disk during recording or reading.
Conventional prior art optical recording actuators having two degrees of freedom of motion (focus and tracking), such as shown in FIG. 1, cannot actively control tilt of the objective lens with respect to the disk and, therefore, cannot correct for the coma. The presence of coma limits the overall system performance and reduces recording quality.